Method of producing nanoparticles using a evaporation-condensation process with a reaction chamber plasma reactor system

ABSTRACT

The present invention provides a method and apparatus for the controlled synthesis of nanoparticles using a high temperature process. The reactor chamber includes a high temperature gas heated by means such as a plasma torch, and a reaction chamber. The homogenizer includes a region between the reactant inlets and the plasma (the spacer zone) to ensure that feeds from the reactant inlets are downstream of the recirculation zone induced by the high temperature gas. It also includes a region downstream of the reactant inlets that provides a nearly I dimensional (varying only in the axial direction) flow and concentration profile in the reaction zone to produce nanoparticles with narrow size distribution.

CROSS-REFERENCE TO RELATED APPLICATIONS

Cross-reference is made to U.S. provisional application No. 60/434158filed on Dec. 17, 2002, which is incorporated herein by reference in itsentirety.

FIELD OF THE INVENTION

The present invention provides a method and apparatus for the controlledsynthesis of nanosize particles using a high temperature process. Thereactor chamber of the present invention includes a high temperatureheating means such as a plasma torch, and a novel reaction chamber. Thereaction chamber is a portion of the reactor chamber having a regionbetween the hot gas inlet and the reactant inlets (the spacer zone) toensure that feeds from the reactant inlets enter the reactor chamberdownstream of the recirculation zone induced by the high temperature gasdischarge. This shift in the location where the reactant gas iscontacted by a hot carrier gas provides a nearly 1-dimensional (varyingonly in the axial direction) flow and concentration profile in thereaction zone yielding nanoparticles having narrow size distribution.

BACKGROUND OF THE INVENTION

The scientific and technological issues of nanostructured particles andmaterials are currently attracting considerable attention. The smallsize of nanoparticles (generally used to indicate particles less than100 nm), which can be responsible for the different properties(electronic, optical, electrical, magnetic, chemical, and mechanical) ofnanoparticles and nanostructured materials with respect to the bulkmaterial, makes them suitable for new applications. Nanosized powdershave been synthesized by a number of processes including colloidalprecipitation, mechanical grinding and gas phase nucleation and growth.Most synthesis methods of nanoparticles in the gas phase are based onhomogeneous nucleation in the gas phase and subsequent condensation andcoagulation. The gas phase synthesis route (aerosol route) makes itpossible to generate new nanoparticles and nanostructured new materialsfrom, in principle, a nearly unlimited variety of starting materials.However, two challenges need to be addressed for nanoparticles producedby aerosol process in order to be suitable for various applications,namely, (1) controlled size distribution of the primary particles, and(2) degree of aggregation, which has a direct effect on dispersibility.For most of the applications, it is very difficult to obtain the desiredproperties when the nanoparticles employed are either widely distributedin primary particle size or highly aggregated or both. Therefore, it isimportant to control the process parameters such as pressure,temperature, and concentration that aid in the determination of theproperties of the resulting particles.

Jet expansion is a convenient fluid mechanical configuration for thecontrolled generation of ultrafine particles by gas-to-particleconversion. Condensable vapor can be introduced into the jet byevaporation from a solid or liquid into the gas upstream from the jet,or by chemical reaction in the jet. The jet configuration permitsparticle production with high throughputs under controlled conditions oftemperature and dilution. U.S. Pat. Nos. 5,935,293 and 5,749,937 toDetering et al., teach a fast quenching reactor and method for thermalconversion of reactants to desired end products such as solid particles.The rapid quenching was achieved by adiabatic and isentropic expansionof gases in a converging-diverging nozzle. By converging-diverging ismeant a nozzle whose area changes in the axial direction, first reducing(“converging”) to a minimum (“throat”), then increasing (“diverging”).Under sufficiently high pressure gradients, the flow velocity willincrease with axial location, reaching a Mach Number of 1 at the throatand increasing to greater than 1 in the diverging section. The expansiontaught can result in cooling rate exceeding 10¹⁰° C./s, thus preservingreaction products that are in equilibrium only at high temperatures.U.S. Pat. Nos. 5,788,738 and 5,851,507 to Pirzada et al., teachessimilar approaches to the production of nanoscale powders by ultra-rapidthermal quench processing of high-temperature vapors through aboundary-layer converging-diverging nozzle, which is an adiabaticexpansion process. The vapor stream is rapidly quenched at rates of atleast 1,000° C./s, preferably greater than 10⁶° C./s, to inhibit thecontinued growth of the nucleated particles and produce nanosize powderof narrow size distribution. One common feature of Detering et al., andPirzada et al.'s work is that the sole purpose of employing a nozzlethat is of a converging-diverging shape is to achieve rapid quench thatis at least greater than 1,000° C./s, preferably greater than 10⁶° C./sby hypersonic nozzle expansion.

U.S. Pat. No. 5,935,293 to Rao et al., teaches a method of producingnanostructured material by hypersonically expanding a particle-gasmixture through a convergent nozzle and directing the resulting jetagainst an impaction substrate. Similar work has been described wherenanosize particles with a narrow size distribution were generated bysubsonically expanding thermal plasma carrying vapor-phase precursorsthrough a convergent nozzle of a similar shape.

A serious difficulty with the jet expansion as taught in the prior artis that these techniques require large pressure gradients to acceleratethe flows; necessitating large and expensive pumps. All theaforementioned nozzles are operated at downstream pressure (gas pressureexiting the nozzle) lower than 760 torr, often considerably lower.

In addition, the discharging of hot gas into an open domain, such as aplasma gas entering a reaction chamber, results in a jet that willentrain local fluid, causing a recirculation region. Any reacting gas orparticles so entrained in the recirculation zone will be exposed,possibly on multiple occasions, to the high temperature gas. This maygreatly accelerate aggregation, sintering and coalescence of theparticles, all of which are generally undesirable. Although not all ofthe reactant gas and particles may be entrained in the recirculationregion induced by the hot gas discharge, the agglomerates formed duringrecirculation will enhance agglomerate formation downstream of therecirculation region through Brownian and turbulent collisions. Thesurprising advantages achieved in separating the location of thereactant inlets upstream of the point where hot carrier gas and thereactants gas come in contact with one another results in this point ofcontact being downstream of the recirculation in the region. The priorpatent literature in this area has failed to teach the novel processresults that can be achieved through the simple nozzle design of thepresent invention.

Presently, the teachings in the patent literature consider nozzle flowonly in the thermodynamic sense; i.e., that the accelerating flow in thenozzle, when there is a sufficient pressure drop through the nozzle(which is controlled by the exit pressure), leads to lower dynamictemperature and pressure, hence leading to lower collision andcoalescence rates. The issue here is not the nozzle per se, but thelarge pressure drop required for such flow, which can be expensive tomaintain and difficult to scale. For example, accelerating the flow to aMach number of 1, which requires a pressure drop of nearly a factor of2, reduces the gas temperature to 75% of the stagnation temperature.This is generally the case for γ˜5/3, where γ is defined as the ratio ofthe specific heat at constant pressure to the specific heat at constantvolume. The ratio of specific heat at constant pressure to the specificheat at constant volume, γ, is usually between 7/5 for diatomic gasesand 5/3 for monatomic gases. Further cooling requires supersonic flowwith substantially greater pressure drops. Since the temperature dropcomes from isentropic adiabatic cooling, special precautions must betaken during the quench step to avoid recovering the temperature whenslowing down the particles to subsonic velocities. The present inventiondemonstrates that nozzle-type flow can be used to produce nanosizedparticles without the need for thermodynamic cooling; the nozzle isoperating under nearly isobaric conditions, which can be definedthermodynamically as the pressure ratio between the exit and inlet ofthe nozzle being less than 0.85, leading to Mach numbers of under 0.40.

Therefore, the present invention satisfies a need of developing acost-effective high temperature aerosol process that is capable ofmaking various types of nanopowders of narrow size distribution. Theinventors have accomplished their desired result to invent acost-efficient reactor and process that produces nanoparticles of theabove described narrow size distribution for a variety of materials bycontrolling the fundamental fluid dynamics in the reactor, especially inthe high temperature region, taking into consideration the recirculatingflow and turbulent diffusion that may occur in the region between thehot gas inlets) and the reactants inlet(s). Thermodynamic cooling asdescribed in the patent literature can be used in conjunction with thisinvention to further improve the particle size distribution.

The main objective of this invention relates to a high temperatureapparatus (aerosol reactor) useful for producing nanoparticles that areeasily dispersed (with a small degree of aggregation, less than 50primary particles in an aggregate after the dispersion step, withprimary particles that are narrowly distributed in size of about 10 nmand 100 nm, preferably between 10 nm and 50 nm and a BET surface areaequal to or greater than about 10 m²/g). Nanoparticles are formed byinjecting the reactants into a high temperature reaction chamber,followed by vapor phase reaction, gas-phase nucleation and subsequentparticle growth by condensation and coagulation. The reaction zonecontains a unique reaction chamber that is precisely designed to reducegas and particle entrainment in the reactant inlets region and topromote efficient mixing in the region downstream of the reactantinlet(s). These features are the key to produce less aggregatednanoparticles with narrow size distribution.

This apparatus can be used for producing novel nanoparticles andnanophase materials by a high temperature aerosol process either with orwithout a chemical reaction using any type of energy source.

SUMMARY OF THE INVENTION

The present invention is a reactor for the production of nanoparticlesin an aerosol process comprising:

-   -   (a) a reaction chamber having a wall, an inlet and an outlet the        inlet for introducing a hot carrier gas to the reaction chamber        which hot carrier gas flows from the inlet through the reaction        chamber and out the outlet,    -   (b) a quench zone located downstream of the reaction chamber        having a quench zone inlet and a quench zone outlet,    -   (c) one or more quench inlets being positioned approximately        about the outlet of the reaction chamber for introducing a        quench material,    -   (d) one or more reactant inlets positioned between the reaction        chamber inlet and the quench zone inlets for introducing one or        more reactants;

the reaction chamber comprising: (i) a spacer zone having a length, L₁,extending from the reaction chamber inlet and ending approximately aboutthe reactant inlets and (ii) a homogenization zone having a length L₂extending from approximately the location of the reactant inlets andending approximately about the quench zone inlet; the spacer zone forallowing the hot carrier gas to allow flow reattachment and carry thereactants to the homogenization zone, the homogenization zone forcontacting the reactants under conditions suitable for forming areaction product and passing the reaction product to the quench zone, L₁being sufficient for the hot carrier gas to attach to the wall of thespacer zone of the reaction chamber prior to the reactant inlets and L₂being sufficient for a residence time of the reactants within thehomogenization zone suitable for forming the reaction product which whenwithdrawn from the outlet of the quench zone is a nanoparticle.

The present invention also discloses an aerosol process for producingnanosize particles, comprising:

(a) introducing a hot carrier gas into an aerosol reactor, the aerosolreactor comprising a reaction chamber and a quench zone having an inletand an outlet, the reaction chamber having a wall, a carrier gas inletand an outlet, one or more quench material inlets being positionedapproximately about the outlet of the reaction chamber, one or morereactant inlets positioned between the carrier gas inlet and the quenchmaterial inlets; the reaction chamber having: (i) a spacer zone having alength, L₁, extending from the reaction chamber inlet and endingapproximately about the reactant inlets and (ii) a homogenization zonehaving a length L₂ extending from approximately the location of thereactant inlets and ending approximately about the quench zone inlet;wherein the hot carrier gas is introduced to the reaction chamber at thecarrier gas inlet, the hot carrier gas flowing through the reactionchamber and out the outlet into the quench zone;

(b) introducing one or more reactants into the reaction chamber at thereactant inlets, the reactants contacting the hot carrier gas in thespacer zone and passing to the homogenization zone to form a reactionproduct, L₁ being sufficient for the hot carrier gas to attach to thewall of the spacer zone of the reaction chamber prior to the reactantinlets and L₂ being sufficient for a residence time of the reactantswithin the homogenization zone suitable for forming the reactionproduct;

(c) passing the reaction product to the quench zone; and

(d) withdrawing from the outlet of the quench zone a nanoparticlereaction product.

Additionally, the present invention is a reactor for the production ofnanoparticles from an aerosol process comprising:

(a) a reactor chamber having axially spaced inlet and outlet ends alongthe reactor axis wherein positioned at the inlet end of the reactorchamber is a high temperature heating means to heat a carrier gas havinga flow direction axially from the reaction chamber inlet downstreamthrough the reaction chamber and out the chamber outlet and wherein oneor more quench gas inlets are positioned up stream from the outlet endof the reactor chamber for introducing a quench gas for cooling;

(b) a reaction chamber having an axially spaced entrance and an exitwherein in the vicinity of the exit, the homogenizer converges to nozzletip, the entrance of the homogenizer being aligned with the inlet to thereaction chamber and the homogenizer being inserted within the reactionchamber and held in place by a homogenizer holder such that thehomogenizer extends from the inlet end of the reaction chamber securelyfitting against the inlet end for at least a portion of thehomogenizer's overall length and wherein the homogenizer comprising twozones: (i) a spacer zone having a length, L₁, extending from thereaction chamber chamber entrance and ending where one or more reactantinlet tubes' are positioned, after having passed through a wall of thereaction chamber, to deliver one or more reactants into the reactionchamber so the reactants contact the hot carrier gas and (ii) ahomogenization zone extending from the reactant inlet tubes' location toa position down stream of the quench gas inlets; and wherein carrier gasand reactants mix and react in the homogenization zone and pass throughthe flow homogenization exit nozzle to enter a quench zone of thereaction chamber defined by the quench gas inlet location in a reactionchamber wall and the reaction chamber outlet and wherein L₁ of thespacer zone must be long enough to have the hot gas flow attached towalls of the reaction chamber before the hot gas reaches the reactantinlets and the overall length (L₁+L₂) of the reaction chamber isdesigned to a residence time sufficient that the following three tasksare completed before gas flow exiting the homogenizer: (1) gas flow inthe reaction chamber has achieved a one-dimensional flow andconcentration profile; and (2) gas-phase nucleation of product particleshas been initiated.

This invention also provides an aerosol process for producing nanosizeparticles, comprising the steps:

(a) introducing a carrier gas into a reactor chamber having (i) axiallyspaced inlet and outlet ends along the reactor axis wherein positionedat the inlet end of the reactor chamber is a high temperature heatingmeans to heat a carrier gas having a flow direction axially from thereaction chamber inlet downstream through the reaction chamber and outthe chamber outlet and wherein one or more quench gas inlets arepositioned up stream from the outlet end of the reactor chamber forintroducing a quench gas for cooling; and (ii) a reaction chamber havingan axially spaced entrance and an exit wherein in the vicinity of theexit, the homogenizer converges to nozzle tip, the entrance of thehomogenizer being aligned with the inlet to the reaction chamber and thehomogenizer being inserted within the reaction chamber and held in placeby a homogenizer holder such that the homogenizer extends from the inletend of the reaction chamber securely fitting against the inlet end forat least a portion of the homogenizer's overall length and wherein thehomogenizer comprising two zones: (i) a spacer zone having a length, L₁,extending from the reaction chamber chamber entrance and ending whereone or more reactant inlet tubes are positioned, after having passedthrough a wall of the reaction chamber, to deliver one or more reactantsinto the reaction chamber so the reactants contact the hot carrier gasand (ii) a homogenization zone extending from the reactant inlet tubes'location to a position down stream of the quench gas inlets; and whereincarrier gas and reactants mix and react in the homogenization zone andpass through the flow homogenization exit nozzle to enter a quench zoneof the reaction chamber defined by the quench gas inlet location in areaction chamber wall and the reaction chamber outlet and wherein L₁ ofthe spacer zone must be long enough to have the hot gas flow attached towalls of the reaction chamber before the hot gas reaches the reactantinlets and the overall length (L₁+L₂) of the reaction chamber isdesigned to a residence time sufficient that the following three tasksare completed before gas flow exiting the homogenizer: (1) gas flow inthe reaction chamber has achieved a near one-dimensional flow andconcentration profile; and (2) gas-phase nucleation of product particleshas been initiated;

(b) heating the carrier gas by thermal contact with the heating means toa temperature to initiate reaction of the carrier gas with one or morereactants;

(c) introducing one or more reactants through the reactant inlet tubesto form a reaction mixture;

(d) adjusting flow rates of the carrier gas and reactants such thatreaction mixture flows through the flow homogenization chamber at a ratesuch that (1) flow of the reaction mixture is characterized byone-dimensional flow and a one-dimensional concentration profile; and(2) gas-phase nucleation of the product has been initiated;

(e) immediately injecting quench gas through the quench gas inlet tubesas the reaction mixture flow enters the quench zone so that particlecoagulation and coalescences is reduced and temperature of the reactionmixture and product is decreased; and

(f) separating and collecting the product.

This invention also provides a reaction chamber for minimizing flowrecirculation in a reactor, the reaction chamber comprising an axiallyspaced entrance and an exit wherein in the vicinity of the exit thehomogenizer converges to nozzle tip, the entrance of the homogenizerbeing aligned with the inlet to the reaction chamber and the homogenizerbeing inserted within the reaction chamber and held in place by ahomogenizer holder such that the homogenizer extends from the inlet endof the reaction chamber securely fitting against the inlet end for atleast a portion of the homogenizer's overall length and wherein thehomogenizer comprising two zones: (i) a spacer zone having a length, L₁,extending from the reaction chamber chamber entrance and ending whereone or more reactant inlet tubes are positioned, after having passedthrough a wall of the reaction chamber, to deliver one or more reactantsinto the reaction chamber so the reactants contact the hot carrier gasand (ii) a homogenization zone extending from the reactant inlet tubes'location to a position down stream of the quench gas inlets; and whereincarrier gas and reactants mix and react in the homogenization zone andpass through the flow homogenization exit nozzle wherein L₁ of thespacer zone must be long enough to have the hot gas flow attached towalls of the reaction chamber before the hot gas reaches the reactantinlets and the overall length (L₁+L₂) of the reaction chamber isdesigned to a residence time sufficient that before gas flow exits thehomogenizer: gas flow in the reaction chamber has achieved a nearone-dimensional flow and concentration profile.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified cutaway diagram of a plasma reactor system fornanoparticle synthesis in accordance with the present invention. In thisFigure only one reactant inlet is illustrated at 104.

FIG. 2 a is a simplified schematic diagram of the reaction chamber. FIG.2 b is a top view of the reaction chamber showing a preferred placementof the reactant inlets.

FIG. 3 is a SEM micrograph of TiO₂ nanoparticles formed under thecondition described in Example A.

FIG. 4 is a SEM micrograph of TiO₂ nanoparticles formed under thecondition described in Example 1.

FIG. 5 is a SEM micrograph of TiO₂ nanoparticles formed under thecondition described in Example 2.

FIG. 6 is a SEM micrograph of TiO₂ nanoparticles formed under thecondition described in Example 3.

FIG. 7 is a SEM micrograph of TiO₂ nanoparticles formed under thecondition described in Example 4.

DETAILED DESCRIPTION

The plasma reactor system of this invention can be used to make titaniumdioxide (titania) nanoparticles and other nanoparticles via eitherreduction or oxidation processes. In the present invention, the termcarrier gas is defined as a gas or gas vapor stream that is heatedbefore entering the reaction chamber by the high temperature heatingmeans. Thus referring to FIG. 1, the carrier gas is the gas or gasmixture that enters the reactor chamber via 16. The carrier gas may be amixture of an inert gas and at least one reactant. For example, in theuse of the present invention to make TiO₂ nanoparticles, the carrier gasmay be argon alone or a mixture of argon and oxygen, or any inert gas orinert gas and oxygen. In the present invention the term “reactantinlets” are a means to introduce at least one reactant into the reactionchamber. The reactant may be mixture of one or more reactant gases orvapors with or without an inert gas, where reactants include at leastone or a mixture of reaction agent compounds that are required to makethe desired product. It is essential for achieving the desired particlesize distribution that no reaction be initiated between the reactantsbefore the reaction components enter the reaction chamber.

The term “attached” or “attachment” with respect to fluid flow refers toa region where, in moving perpendicular from the boundary wall into thebulk of the fluid, the flow parallel to the boundary does not changesign (that is, the flow parallel to the boundary is moving in the samedirection, varying only in amplitude). The term “separated” with respectto fluid flow refers to a region where, in moving perpendicular from theboundary wall into the bulk of the fluid, the flow parallel to theboundary changes sign. The zone between “separated” flow and “attached”flow is referred as the stagnation point“and represents a singularsolution to the boundary layer fluid equation.

The reaction chamber of the present invention comprises a wall, an inletand an outlet, the inlet for introducing a hot carrier gas to thereaction chamber, and the hot carrier gas flows from the inlet throughthe reaction chamber and out the outlet. It further comprises ahomogenizer which provides the spacer zone and the homogenization zone.The homogenizer can be made of any suitable material, with copper orceramic materials being preferred.

A feature of this invention is a reaction chamber that is used in a hightemperature aerosol reactor for the controlled synthesis ofnanoparticies. This reaction chamber promotes near one-dimensional flowand concentration profiles by enhanced mixing of the reactants andcarrier gas as these gases flow down stream through the spacer zone, thehomogenization zone, and into the quench zone. The reaction chamber canbe used with very small pressure gradients.

Throughout the Figures herein, recurring elements are designated with bythe same number and symbol. A plasma reactor system according to thepresent invention (a nanoparticle generating reactor or aerosol reactor)10 is schematically shown in FIG. 1. The reaction chamber 26 isschematically shown in FIG. 2 a.

In FIG. 1, the reactor consists of a high temperature energy source 24,reaction chamber 26 (also shown in FIG. 2 a), quenching chamber 30 andproduct collector 32. Each of these regions of the reactor chamber iscooled by fluid circulating within the walls of the reactor chamber (notshown). The preferred cooling fluid for use in the present invention iswater.

In a preferred embodiment, the energy source 24 is a DC arc plasmatorch. As shown in FIG. 1, the carrier gas is supplied from tank 14through line 16 to the energy source 24. The heating source is alsocooled by a cooling fluid circulation through a cooling jacket (notshown). The preferred coolant is water. The reaction chamber of thepresent invention comprises a wall 28, an inlet 50 and an outlet 56, theinlet for introducing a hot carrier gas to the reaction chamber, and thehot carrier gas flows from the inlet through the reaction chamber andout the outlet. It further comprises a homogenizer which provides thespacer zone 52 and the homogenization zone 54.

The reaction chamber may be made of any material of construction that issuitable for use in high temperature, oxidizing and/or corrosiveenvironments. High purity alumina can be employed. It may be made of amaterial of construction that meets the following requirements: a goodheat insulator; able to withstand temperatures that can be achievedusing plasma heating; able to survive thermal shock; able to surviveoxidizing and reducing environments depending on the application; andable to survive a corrosive environment. The homogenizer can be made ofany suitable material, with ceramic materials being preferred.

The reactants, for example titanium tetrachloride and oxygen, areinjected through line 20 into the reaction chamber through inlet 104(preferably three equally-spaced radial inlets) as vapor in a carriergas (generally oxygen) by first bubbling oxygen housed in cylinder 12through line 18 into a liquid reactant TiCl₄ stored in 36, and thenthrough line 20 into the reaction chamber. On entering the reactionchamber and contacting the hot carrier gas flow from the energy source,the reaction is initiated and continues as the reactants flow downstreamtoward reaction chamber exit 56, and into the quench zone, into thequenching chamber 30, where quenching gas 22 from tank 12 is radiallyintroduced into the the quench chamber through inlets 110. Additionally,the temperature of the aerosol stream is reduced by mixing with thequenching gas. As a result the rates of particle coagulation andaggregation are reduced. Further downstream the particles are collectedin the product collector 32. In the present example, a sintered metalfilter is used to collect the product, although any suitable collectiondevice, made of a suitable material, could be used. The gas flow exitingthe filter is discharged into a scrubber 34. In one embodiment of thisprocess, primary particles in the sub-50 nm range are formed with thereaction chamber.

As shown in FIG. 2 a, the reaction chamber consists of two zones. A zonebetween the hot gas inlet 50, having diameter D₁, and one or morereactant inlets 104 is the spacer zone 52, having an upper diameter D₂,converging to a lower diameter D₃ at the reactant inlets, and has lengthL₁. The region between the reactant inlets 104 and the quench chamber 56inlet is the homogenization zone 54, having length L₂. The spacer zonelength L₁ must be long enough to have the hot gas flow attached beforereaching the reactant inlets. The flow detachment is caused by expandingthe hot gas into the spacer region as a free jet, thus inducing flowrecirculation. The optimal length of the spacer zone is dependent on thetemperature and flow rate of the hot gas, the hot gas inlet 50 withdiameter D₁ and the diameter of the reactant inlet region 60 D₃. Makingthe spacer zone any longer is at the expense of wasting high temperatureenergy. The homogenization zone has an initial tubular region followedby a first converging section 62. The homogenizer is designed to have aminimum residence time so that the following tasks are completed beforethe gas flow exiting the homogenizer: (1) creation of one-dimensionalflow and concentration profile; (2) initiation of gas-phase nucleation.This serves as the base of determining the length of the homogenizationzone L₂, and the diameters D₃ and D₄, the diameter of the entrance tothe quench chamber. Therefore, the dimensions are calculated based onthe reaction rate, rate of mixing induced by diffusion and turbulenceand nucleation rate. Increasing the flow residence time by increasingthe volume of the homogenization zone for fixed flow rate is notadvantageous. Once the nuclei are formed the aerosol stream should bequenched immediately so that the particle growth by coagulation andcoalescence can be reduced as the temperature decreases. Therefore, aminimum length for the homogenization zone is preferred. Experimentationor calculation may determine the optimal length of the zone with respectto the particular product desired and the process conditions.

In FIG. 2 a, a straight extension section of length L₃ may optionally beadded to the end of the reaction chamber at 56 to adjust final productproperties. The length of this zone, L₃, does not seem critical. Theextended zone may be needed for achieving the desired taper for thenozzle tip or for mechanical reasons, for example.

The reactant(s) are injected directly radially into the reactionchamber. FIG. 1 illustrates one inlet 104 and FIG. 2 b, a cross-sectionof the reaction chamber inlet, illustrates 3 equally-spacedradially-distributed inlets. It is preferable to have multiple inlets.

A high temperature heating means (24) is employed in the presentinvention. Non-limiting examples of the heating means employed includeDirect Current (DC) arc plasma, Radio Frequency (RF) plasma, electricheating, conductive heating, flame reactor and laser reactor.Particularly useful means in the present invention are DC arc plasma andRF plasma.

A reactant stream (20) is employed in the present invention. The streamcan be in liquid, solid, vapor, emulsion, dispersion, solution or powderform, or any combination thereof. Non-limiting examples of feedmaterials include solid particles carried by an inert gas, a reactantgas or combination thereof; a solid precursor positioned inside theheating zone; liquid droplets carried by an inert gas, a reactant gas orcombination thereof; vapor phase precursor carried by an inert gas orreactant gas or combination thereof, wherein the vapor phase precursoris a suspension of solid particles and liquid droplets that aregenerated by an auxiliary apparatus and fed into the apparatus andprocess of the current invention. Sizes of particles and liquid dropletsmay be of any useful size.

The shape and dimension of the reaction chamber is predetermined by bothexperiment and modeling in order to obtain the desired fluid dynamicsfeature.

A reactant inlet (104) is comprised of a tube, and is employed in thepresent invention. This tube can be made of any material of constructionthat can survive a corrosive environment, or any other environmentdetermined by the reactants. The diameter of the tube must be smallenough so that high velocities of the reactants are achieved, therebyallowing the reactants to penetrate into the high temperature plasma.The diameter of the tube is determined by the flow rate and desiredturbulence.

In summary, the present invention may be distinguished from apparati andprocesses currently known.

The reaction chamber described in the current invention includes astraight region and a convergent section, whereas the nozzles describedin U.S. Pat. Nos. 5,935,293 and 5,749,937 by Detering et al., and U.S.Pat. Nos. 5,788,738 and 5,851,507 by Pirzada et al. all have adivergent-convergent shape.

One of the design features of the reaction chamber is to inject thereactants a certain distance downstream from the carrier gas inlet toavoid exposing the reactant(s) to the flow recirculation induced by thehot gas discharging into the reaction chamber. This issue is notaddressed in U.S. Pat. Nos. 5,935,293 and 5,749,937 by Detering et al.,U.S. Pat. Nos. 5,788,738 and 5,851,507 by Pirzada et al., and U.S. Pat.No. 5,874,134 by Rao et al.

The present invention relates to a high temperature process comprising aunique reaction chamber that is designed to reduce flow recirculation inthe region between the hot gas inlet(s) and the reactant inlet(s), andto promote efficient mixing in the region downstream of the reactantsinlet(s). As a result of the enhanced mixing, the concentration profileof the product vapor in the homogenizer approaches one-dimension. Thus,nanoparticles with narrow size distribution can be produced.

The process requires a relatively uniform flow profile (i.e., nearlyone-dimensional) to aid the formation of narrowly distributed primarynanoparticles, and to prevent recirculation that can promote theformation of hard aggregates. The uniform concentration profile createdby the homogenizer enables the nucleation to take place in a veryuniform and controlled manner, thus allowing the formation ofnanoparticles with relatively narrow particle size distribution.

The present process for producing nanoparticles can be applied toprecursors such as solids, liquids, and vapors.

The current invention is also aimed at promoting efficient mixing sothat particles can be formed in a more uniform manner. It can beoperated subsonically (defined as Mach number<0.5) and the coolingeffect created by the expansion is negligible. U.S. Pat. Nos. 5,935,293and 5,749,937 by Detering et al., U.S. Pat. Nos. 5,788,738 and 5,851,507by Pirzada et al., aim at obtaining rapid quench via supersonicexpansion through a nozzle.

Additionally, the gas pressure at the exit of the reaction chamber canbe in the range of 1-5 atmosphere, whereas the applications elsewheredescribed (U.S. Pat. Nos. 5,935,293 and 5,749,937 by Detering et al.,U.S. Pat. Nos. 5,788,738 and 5,851,507 by Pirzada et al., and U.S. Pat.No. 5,874,134 by Rao et al.) all require substantial pressuredifferential and the pressure at the nozzle exit are well belowatmospheric pressure. The apparatus discussed here can be operated usingthe nozzle with a large pressure gradient to achieve thermodynamiccooling to further improve particle size distribution.

It will be recognized by those skilled in the art of reactor design andmodeling that the reaction chamber of the present invention is useful ina variety of reactors in addition those reactors for producing nanosizeparticles.

Titanium dioxide nanoparticles made according to the present inventionmay be used with advantage in various applications including sunscreenand cosmetic formulations; coatings formulations including automotivecoatings, wood coatings, and surface coatings; chemical mechanicalplanarization products; catalysis products including photocatalysts usedin water and air purification and selective catalytic reduction catalystsupports; photovoltaic cells; plastic parts, films, and resin systemsincluding agricultural films, food packaging films, molded automotiveplastic parts, and engineering polymer resins; rubber based productsincluding silicone rubbers; textile fibers, woven and nonwovenapplications including polyamid, polyaramide, and polyimides fibersproducts and nonwoven sheets products; ceramics; glass productsincluding architectural glass, automotive safety glass, and industrialglass; electronic components; and other uses The following Examples, arenot intended to limit the present invention, but to illustrate at leastsome of the benefits of the present invention.

Test Methods

The analytical methods that are listed in the Table are BET surfacearea, UPA particle size distribution and SAXS particle sizedistribution. These techniques are briefly described in the followingsection.

BET Surface Area

The surface areas of powders and solids are calculated using theadsorption of nitrogen at its boiling point via the BET method, S.Brunauer, P. H. Emmett, and E. Teller, JACS 60, 309 (1938). AMICROMERITICS ASAP 2405 (a trademark of Micromeritics, Inc., Atlanta,Ga.) adsorption apparatus is used to measure the amount of nitrogensorbed; the BET equation is used to calculate the amount of nitrogencorresponding to a monolayer for a given sample. Using an area of 16.2Å² per nitrogen molecule under the sorption conditions, the surface areaper gram of solid is calculated. Surface area standards from theNational Institute of Standards & Technology are run to insure that thereported values are accurate to within a few percent. For non-poroussolids (nearly spherical or cubical), the BET surface area can becompared with the size obtained from another technique (e.g. microscopicor particle size analysis). The relationship is${SA} = \frac{6}{\rho*D}$where SA is the surface area in m²/g, p the density in g/cc, and D thediameter in microns (μm). This relationship is exact for spheres andcubes. Therefore, the higher the surface area the smaller the particlesize.

UPA Particle Size Distribution

The MICROTRAC ULTRAFINE PARTICLE ANALYZER (UPA) (a trademark of Leedsand Northrup, North Wales, Pa.) uses the principle of dynamic lightscattering to measure the particle size distribution of particles inliquid suspension. The instrument is manufactured by Leeds and Northrup,North Wales, Pa. The measured size range is 0.003 μm to 6 μm (3 nm to6000 nm). The dry particle sample needs to be prepared into a liquiddispersion to carry out the measurement. An example procedure is asfollow:

(1) Weigh out 0.08 g dry powder.

(2) Add 79.92 g 0.1% tetrasodium pyrophosphate (TSPP) solution in waterto make a 0.1 wt. % suspension.

(3) Sonify the suspension for 10 minutes using an ultrasonic probe. Thesuspension should be cooled in a water-jacketed beaker duringsonication.

(4) When sonication is complete, draw an aliquot for analysis.

SAXS Particle Size Distribution

In principal, SAXS measures “electron density” and then calculates thesize of an equivalent spherical particle with the measured electrondensity. The SAXS intensity at a particular angle depends on theelectron density contrast between the particle (e.g., TiO₂) and thesurrounding medium (e.g., air). It also depends on the size of theparticles. Large particles scatter mostly at low angles and smallparticles scatter at larger angles.

The powders are dusted on a piece an adherent substrate. Some of thepowder adhere to the substrate, which is mounted on a sample holder. Thex-rays (wavelength 0.154 nm, CuKalpha) are produced by standardgenerators. Two sets of data are collected that cover overlapping rangesin scattering angle. A Kratky instrument is used to collect small-anglescattering at the larger scattering angles (1e-1 to 4e-0 degrees). ABonse Hart instrument is used to collect small-angle scattering at thesmaller scattering angles (2.2e-3 to 5e-1 degrees). The two datasets arecombined into a single scan after background subtraction, and the dataare subsequently desmeared. These desmeared data are then transformed toa volume size distribution function by the regularization technique. Thevolume distribution function is the final output of this procedure.

EXAMPLES

Unless otherwise specified, all chemicals and reagents were used asreceived from Aldrich Chemical Co., Milwaukee, Wis.

Example A

TiCl₄ vapor was thoroughly premixed with O₂ by bubbling O₂ at a rate of10 l/min through a cylinder maintained at room temperature that containsliquid TiCl₄. The mixture of TiCl₄ and O₂ was then introduced into thereaction chamber through three equally spaced radial ports that are 0.32cm in diameter. The reaction chamber was of cylindrical shape (2.52 cmin diameter, 7.56 cm in height) and did not contain a homogenizer.Gaseous titanium dioxide was formed by TiCl₄ oxidation reaction. TiO₂aerosol particles were formed by gas-phase nucleation of the TiO₂ vaporfollowed by condensation and coagulation. At the end of the reactionchamber, room temperature O₂ was introduced radially into the quenchingchamber at a rate of 30 l/min where the high temperature aerosol streamwas lowered by mixing with the room temperature quenching gas. Thequenching chamber is of cylindrical shape (2.52 cm in diameter, 20.16 cmin height). Downstream from the quenching chamber, TiO₂ particles werecollected by a sintered metal filter. The properties of the resultingTiO₂ particles are listed in the Table. FIG. 3 is a SEM micrograph ofthe TiO₂ nanoparticles produced under this condition.

Example 1

The process of Example A was repeated except that (1) a homogenizer (asshown in FIG. 2 a) was installed in the reaction chamber (; (2) thediameter of the TiCl₄ injection ports was reduced to 0.02 cm. Thedimensions of the homogenizer and the properties of the resulting TiO₂particles are listed in the Table. FIG. 4 is an SEM micrograph of theTiO₂ nanoparticles produced under this condition.

Example 2

The process of Example 1 was repeated except that L₁ of the homogenizeris 0.9 cm. Accordingly, the TiCl₄ injection ports were moved upstream by3.8 cm to the hot gas inlet. The dimensions of the reaction chamber andthe properties of the resulting TiO₂ particles are listed in the Table.FIG. 5 is an SEM micrograph of the TiO₂ nanoparticles produced underthis condition.

Example 3

The process of Example A was repeated except a straight section that was5.6 cm long was added to the reaction chamber (L₃ shown in FIG. 2). Thedimensions of the reaction chamber and the properties of the resultingTiO₂ particles are listed in the Table, below. FIG. 6 is an SEMmicrograph of the TiO₂ nanoparticles produced under this condition.

Example 4

The process of Example A was repeated with a shortened homogenizer (L₂is 2.7 cm) and the diameter of the homogenizer D₄ is 0.95 cm. Thedimensions of the reaction chamber and the properties of the resultingTiO₂ particles are listed in the Table, below. FIG. 7 is an SEMmicrograph of the TiO₂ nanoparticles produced under this condition.TABLE Summary of Homogenizer Dimension and Particle Properties fromExamples Example Number Homogenizer Dimensions A 1 2 3 4 D₁, cm NA 0.80.8 0.8 0.8 D₂, cm NA 2.52 2.52 2.52 2.52 D₃, cm NA 1.5 1.5 1.5 1.5 D₄,cm NA 0.5 0.5 0.5 0.95 L₁, cm NA 4.6 0.9 4.6 4.6 L₂, cm NA 6.7 6.7 6.72.7 L₃, cm NA 0 0 5.6 0 Reactant inlet 0.32 0.02 0.02 0.02 0.02diameter, cm vmd¹, nm 97.0 37.6 51.0 36.0 57.4 vmd², nm 45.3 22.9 36.526.2 31.4 vmd¹/vmd² 2.1 1.6 1.4 1.4 1.83 Surface Area, m²/g 44.7 103.957.6 99.6 73.7 Conversion % 36 32 61 64 981. vmd¹ is volume mean diameter measured by UPA dynamic lightscattering.2. vmd² is volume mean diameter measured by SAXS, small angle x-raydiffraction.3. vmd¹/vmd² is the ratio of volume mean diameters measured by eachmethod.4. Surface area was measured by BET surface absorption.

Based on the results described in the examples and table, the followingobservations are made. The effect of the reaction chamber on the size ofTiO₂ nanoparticles is demonstrated by Example A and Example 1. With thehomogenizer as in Example 1, the BET surface area increases from 44.7m²/g to 103.9 m²/g, suggesting a significant reduction in averageprimary particle size. In the meanwhile the volume median diameterdecreases from 97 nm to 37.6 nm, suggesting that the dispersibleparticle size is substantially reduced. The size uniformity of theprimary particles is demonstrated in FIGS. 3 and 4. In FIG. 3 particlesabove 100 nm and below 30 nm are both observed, while in FIG. 4 the vastmajority of the primary particles are in the range of 10-30 nm.Evidently the reaction chamber can reduce the primary particle size,increase the size uniformity of the primary particles and improve thedispersibility.

The importance of the location of the reactant inlets is demonstrated byExample 1 and 2. When the length of the spacer zone is reduced by 3.78cm, which results in a shorter distance between the reactant inlets andthe hot gas inlet, the BET surface area drops from 103.9 m²/g to 57.6m²/g. In the meanwhile the dispersible volume mean diameter increasesfrom 37.6 nm to 51 nm. The increase in particle size is very likelycaused by the entrainment of the reactant gas in the flow recirculationthat is induced by the high temperature gas entering in the upstreamsection of the homogenizer.

Examples 1 and 3 study the effect of the length of the homogenizer afterthe reaction inlets. In Example 3 a straight section, 5.6 cm long, isadded at the end of the homogenizer. The resulting volume mediandiameter measured by SAXS is increased from 22.9 nm to 26.2 nm. In FIG.6 the SEM micrograph shows that there is more necking between theprimary particles. Thus, if the homogenizer is too long, the nucleusformed by gas-phase nucleation will remain in the homogenizer instead ofbeing exposed to the quenching gas. The high temperature in thehomogenizer will result in more particle sintering.

Examples 1 and 4 also demonstrated the impact of the length of thehomogenization zone. In Example 4, L₂ was reduced from 6.7 to 2.7 cm,the resulting volume particle surface area was decreased from 103.9 m²/gto 73.7 m²/g, and the dispersible volume mean diameter increased from37.6 nm to 57.4 nm.

1. A reactor for the production of nanoparticles in an aerosol processcomprising: (a) a reaction chamber having a wall, an inlet and an outletthe inlet for introducing a hot carrier gas to the reaction chamberwhich hot carrier gas flows from the inlet through the reaction chamberand out the outlet, (b) a quench zone located downstream of the reactionchamber having an inlet and an outlet, (c) one or more quench inletsbeing positioned approximately about the outlet of the reaction chamberfor introducing a quench material, (d) one or more reactant inletspositioned between the reaction chamber inlet and the quench inlets forintroducing one or more reactants; the reaction chamber comprising: (i)a spacer zone having a length, L₁, extending from the reaction chamberinlet and ending approximately about the reactant inlets and (ii) ahomogenization zone having a length L₂ extending from approximately thelocation of the reactant inlets and ending approximately about thequench zone inlet; the spacer zone for allowing the hot carrier gas tocarry the reactants to the homogenization zone, the homogenization zonefor contacting the reactants under conditions suitable for forming areaction product and passing the reaction product to the quench zone, L₁being sufficient for the hot carrier gas to attach to the wall of thespacer zone of the reaction chamber prior to the reactant inlets and L₂being sufficient for a residence time of the reactants within thehomogenization zone suitable for forming the reaction product which whenwithdrawn from the outlet of the quench zone is a nanoparticle.
 2. Thereactor of claim 1, which further comprises a high temperature heatingmeans for heating the carrier gas selected from the group consisting ofa DC plasma arc, RF plasma, electric heating, conductive heating, flamereactor and laser reactor.
 3. The reactor of claim 1, which furthercomprises a DC plasma arc for heating the carrier gas.
 4. The reactor ofclaim 1, which further comprises an RF plasma for heating the carriergas.
 5. The reactor of claim 1, wherein the reaction chamber furthercomprises a homogenizer which provides the spacer zone and thehomogenization zone.
 6. The reactor of claim 5, wherein the homogenizeris constructed of copper or ceramic material.
 7. The reactor of claim 5,wherein the homogenizer has a wall, an entrance and an exit, thehomogenizer wall converging to a nozzle tip at the exit which is spaceda distance L₁+L₂+L₃ from the entrance.
 8. The reactor of claim 7 inwhich the distance L₃ is zero.
 9. An aerosol process for producingnanosize particles, comprising: (a) introducing a hot carrier gas intoan aerosol reactor, the aerosol reactor comprising a reaction chamberand a quench zone having an inlet and an outlet, the reaction chamberhaving a wall, a carrier gas inlet and an outlet, one or more quenchmaterial inlets being positioned approximately about the outlet of thereaction chamber, one or more reactant inlets positioned between thecarrier gas inlet and the quench material inlets; the reaction chamberhaving: (i) a spacer zone having a length, L₁, extending from thereaction chamber inlet and ending approximately about the reactantinlets and (ii) a homogenization zone having a length L₂ extending fromapproximately the location of the reactant inlets and endingapproximately about the quench zone inlet; wherein the hot carrier gasis introduced to the reaction chamber at the carrier gas inlet, the hotcarrier gas flowing through the reaction chamber and out the outlet intothe quench zone; (b) introducing one or more reactants into the reactionchamber at the reactant inlets, the reactants contacting the hot carriergas in the spacer zone and passing to the homogenization zone to form areaction product, L₁ being sufficient for the hot carrier gas to attachto the wall of the spacer zone of the reaction chamber prior to thereactant inlets and L₂ being sufficient for a residence time of thereactants within the homogenization zone suitable for forming thereaction product; (c) passing the reaction product to the quench zone;and (d) withdrawing from the outlet of the quench zone a nanoparticlereaction product.
 10. The process of claim 9, wherein the reactants areTiCl₄ and O₂ and the product is TiO₂ particles.
 11. The titanium dioxideparticles of claim 10 having a particle size of between 10 nm and 100 nmand a BET surface area of more than 10 m²/g.
 12. The process of claim 9,wherein the carrier gas is inert.
 13. The process of claim 9 wherein thecarrier gas is selected from the group consisting of argon, oxygen,nitrogen, and a combination thereof.
 14. The process of claim 9, whereinthe reactants are one or more precursor materials.
 15. The process ofclaim 9, wherein the reactants are in the vapor, liquid, emulsion,dispersion, solution or powder form.
 16. The process of claim 9, whereinthe carrier gas is introduced to the reaction chamber so that it has aflow direction axially from the chamber inlet downstream through thereaction chamber.
 17. A reaction chamber for minimizing flowrecirculation in a reactor, the reaction chamber comprising a wall, anentrance and an exit wherein, in the vicinity of the exit, the wall ofthe homogenizer converges to a nozzle tip from which a reaction productcan be withdrawn, a hot carrier gas inlet located about the entrance ofthe reaction chamber and quench material inlets located about the exitof the reaction chamber and one or more reactant inlets located betweenthe hot carrier gas inlet and the quench inlets, the homogenizer having(i) a spacer zone having a length, L₁, extending from the reactionchamber entrance and ending about the reactant inlets and (ii) ahomogenization zone having a length L₂ extending from the reactantinlets to a position downstream of the quench inlets for contacting thehot carrier gas and the reactants and wherein L₁ of the spacer zone issufficient for the hot carrier gas to attach to the wall of the reactionchamber before the hot carrier gas reaches the reactant inlets and L₂ ofthe reaction chamber being sufficient for a residence time within thehomogenization zone suitable for forming the reaction product.
 18. Areactor for the production of nanoparticles from an aerosol processcomprising: (a) a reactor chamber having axially spaced inlet and outletends along the reactor axis wherein positioned at the inlet end of thereactor chamber is a high temperature heating means to heat a carriergas having a flow direction axially from the reaction chamber inletdownstream through the reaction chamber and out the chamber outlet andwherein one or more quench gas inlets are positioned up stream from theoutlet end of the reactor chamber for introducing a quench gas forcooling; (b) a reaction chamber having an axially spaced entrance and anexit wherein in the vicinity of the exit, the homogenizer converges to anozzle tip, the entrance of the homogenizer being aligned with the inletto the reaction chamber and the homogenizer being inserted within thereaction chamber and held in place by a homogenizer holder such that thehomogenizer extends from the inlet end of the reaction chamber securelyfitting against the inlet end for at least a portion of thehomogenizer's overall length and wherein the homogenizer comprising twozones: (i) a spacer zone having a length, L₁, extending from thereaction chamber chamber entrance and ending where one or more reactantinlet tubes are positioned, after having passed through a wall of thereaction chamber, to deliver one or more reactants into the reactionchamber so the reactants contact the hot carrier gas and (ii) ahomogenization zone extending from the reactant inlet tubes' location toa position down stream of the quench gas inlets; and wherein carrier gasand reactants mix and react in the homogenization zone and pass throughthe flow homogenization exit nozzle to enter a quench zone of thereaction chamber defined by the quench gas inlet location in a reactionchamber wall and the reaction chamber outlet and wherein L₁ of thespacer zone must be long enough to have the hot gas flow attached towalls of the reaction chamber before the hot gas reaches the reactantinlets and the overall length (L₁+L₂) of the reaction chamber isdesigned to a residence time sufficient that the following three tasksare completed before gas flow exiting the homogenizer: (1) gas flow inthe reaction chamber has achieved a near one-dimensional flow andconcentration profile; and (2) gas-phase nucleation of product particleshas been initiated.
 19. An aerosol process for producing nanosizeparticles, comprising the steps: (a) introducing a carrier gas into areactor chamber having (i) axially spaced inlet and outlet ends alongthe reactor axis wherein positioned at the inlet end of the reactorchamber is a high temperature heating means to heat a carrier gas havinga flow direction axially from the reaction chamber inlet downstreamthrough the reaction chamber and out the chamber outlet and wherein oneor more quench gas inlets are positioned up stream from the outlet endof the reactor chamber for introducing a quench gas for cooling; and(ii) a reaction chamber having an axially spaced entrance and an exitwherein in the vicinity of the exit, the homogenizer converges to nozzletip, the entrance of the homogenizer being aligned with the inlet to thereaction chamber and the homogenizer being inserted within the reactionchamber and held in place by a homogenizer holder such that thehomogenizer extends from the inlet end of the reaction chamber securelyfitting against the inlet end for at least a portion of thehomogenizer's overall length and wherein the homogenizer comprising twozones: (i) a spacer zone having a length, L₁, extending from thereaction chamber chamber entrance and ending where one or more reactantinlet tubes are positioned, after having passed through a wall of thereaction chamber, to deliver one or more reactants into the reactionchamber so the reactants contact the hot carrier gas and (ii) ahomogenization zone extending from the reactant inlet tubes' location toa position down stream of the quench gas inlets; and wherein carrier gasand reactants mix and react in the homogenization zone and pass throughthe flow homogenization exit nozzle to enter a quench zone of thereaction chamber defined by the quench gas inlet location in a reactionchamber wall and the reaction chamber outlet and wherein L₁ of thespacer zone must be long enough to have the hot gas flow attached towalls of the reaction chamber before the hot gas reaches the reactantinlets and the overall length (L₁+L₂) of the reaction chamber isdesigned to a residence time sufficient that the following three tasksare completed before gas flow exiting the homogenizer: (1) gas flow inthe reaction chamber has achieved a near one-dimensional flow andconcentration profile; and (2) gas-phase nucleation of product particleshas been initiated; (b) heating the carrier gas by thermal contact withthe heating means to a temperature to initiate reaction of the carriergas with one or more reactants; (c) introducing one or more reactantsthrough the reactant inlet tubes to form a reaction mixture; (d)adjusting flow rates of the carrier gas and reactants such that reactionmixture flows through the flow homogenization chamber at a rate suchthat (1) flow of the reaction mixture is characterized byone-dimensional flow and a one-dimensional concentration profile; and(2) gas-phase nucleation of the product has been initiated; (e)immediately injecting quench gas through the quench gas inlet tubes asthe reaction mixture flow enters the quench zone so that particlecoagulation and coalescences is reduced and temperature of the reactionmixture and product is decreased; and (f) separating and collecting theproduct.
 20. A reaction chamber for minimizing flow recirculation in areactor, the reaction chamber comprising an axially spaced entrance andan exit wherein in the vicinity of the exit the homogenizer converges tonozzle tip, the entrance of the homogenizer being aligned with the inletto the reaction chamber and the homogenizer being inserted within thereaction chamber and held in place by a homogenizer holder such that thehomogenizer extends from the inlet end of the reaction chamber securelyfitting against the inlet end for at least a portion of thehomogenizer's overall length and wherein the homogenizer comprising twozones: (i) a spacer zone having a length, L₁, extending from thereaction chamber chamber entrance and ending where one or more reactantinlet tubes are positioned, after having passed through a wall of thereaction chamber, to deliver one or more reactants into the reactionchamber so the reactants contact the hot carrier gas and (ii) ahomogenization zone extending from the reactant inlet tubes' location toa position down stream of the quench gas inlets; and wherein carrier gasand reactants mix and react in the homogenization zone and pass throughthe flow homogenization exit nozzle wherein L₁ of the spacer zone mustbe long enough to have the hot gas flow attached to walls of thereaction chamber before the hot gas reaches the reactant inlets and theoverall length (L₁+L₂) of the reaction chamber is designed to aresidence time sufficient that before gas flow exits the homogenizer:gas flow in the reaction chamber has achieved a near one-dimensionalflow and concentration profile.